In recent years, there has been known a hybrid work machine having an engine downsized from viewpoints of energy saving and causing an electric storage device and a motor-generator to assist the engine in output to solve the problem of insufficient output resulting from engine downsizing. In such a hybrid work machine, there is a probability that sufficient electric power for assisting the engine cannot be supplied to the motor-generator in a low-temperature environment where it may be necessary to perform a warm-up operation on the engine because the internal resistance of the electric storage device increases and a discharge current falls.
To address such a problem, there is known a hybrid construction machine warm-up method (see, for example, Patent Literature 1) including actuating an engine to perform a warm-up operation thereon, changing a target charge rate of an electric storage device, actuating a motor-generator, and charging or discharging the electric storage device, thereby generating heat from the electric storage device (warming up the electric storage device) when a temperature of the electric storage device is lower than a preset temperature.